In order to enhance a vehicle's aesthetic qualities, it is common to integrate vehicle radio antennas with one or more of the vehicle's windows (commonly known as hidden antenna systems or on-glass AM/FM antenna systems). Unlike mast (i.e. rod) antennas, on-glass antenna systems do not introduce any external vehicle protrusions and they typically offer excellent mechanical stability and satisfactory reception performance. Although on-glass antennas are widely used, they do suffer from various drawbacks. Specifically, on-glass antennas are difficult to design because a small change in a vehicle's body design can radically change the reception performance of the antenna.
On-glass antenna systems are usually fabricated by printing metallic conductors on an inner surface of the back-glass or the side-glass (FIG. 1) of a vehicle window. A low-noise-amplifier (LNA) circuit is typically mounted in close proximity to the on-glass antenna and is electrically coupled to the on-glass antenna to amplify the weak signal received by the antenna before it is sent to the radio receiver for further conditioning. The on-glass antennas are typically fed vertically (close to the vehicle roof) so that the LNA circuit can be housed in the vicinity where the vehicle roof intersects the window.
Referring to FIG. 2, it is also known to locate on-glass antennas on the back glass 3 (i.e. rear windshield glass) of a vehicle, V, to provide an AM/FM antenna system. As illustrated, the rear windshield glass 3 includes a plurality of horizontally-fed wires 4 that function as a defroster grid to melt or snow and ice from the glass 3. Additional grid lines (not shown) that are on or, in close proximity to the horizontal defroster grid lines 4 may also be used as the AM/FM antenna system. However, such additional grid line are generally not adequate to provide a desired signal reception performance.